It's still hugely out of character for Spock, considering how he and his friends reacted to altered timelines in the past. I can't believe he just wouldn't do anything.
If Spock believed in a Linear Timeline, the fact that 25 years of irreversible changes have occurred would tell him otherwise.
Logically, an Alternate Reality is the only logical alternative.
Therefore, travelling back in time changes nothing, and merely spawns another reality.
That is assuming he has the opportunity to even do this.